1. Field of the Invention
The present invention relates to stencil printing machines and more particularly to a stencil printing machine equipped with plural printing drums for achieving a multi-color printing and a method of controlling the same.
2. Description of the Related Art
Various research and development work have been undertaken to provide a stencil printing machine of the type which enables a multi-color printing with the use of a plurality of printing drums, a first typical example of which is disclosed in FIG. 6 which shows a part of a structure of the prior art stencil printing machine. In FIG. 6, first and second printing drums 101 and 102 and a press drum 103 are rotatably supported in a frame body (not shown) such that the first and second printing drums 101 and 102 are located in close proximity to an outer periphery of the press drum 103 and are angled at 90 degrees relative to a central axis of the press drum 103. Outer circumferential peripheries of the respective first and second printing drums 101 and 102 carry thereon stencil clamping bases 101a and 102a, respectively, which support thereon sheet clamping segments 101b and 102b for clamping stencil sheets (not shown) onto the stencil clamping bases 101a and 102a, respectively.
Further, screens 105 are wound on the outer circumferential peripheries of the first and second printing drums 101 and 102 in a stretched state with the use of the stencil clamping bases 101a and 102a and springs 104 and 105, with each of the screens 105 being formed of a porous or mesh structure which allows printing ink to permeate. Inner press rollers 106 and 107 are located inside the screens 105 of the first and second printing drums 101 and 102, respectively, with the inner press rollers 106 and 107 being moveable between a wait position not to press the screens 105 and a press engagement position to press the screens 105.
During printing operation, the inner press rollers 106 and 107 are maintained in the press position, in which the screens 105 are expanded outward. Also, it is arranged such that the screen 105 of the first printing drum 101 is supplied with printing ink in a first color by the inner press roller 106 and the screen 105 of the second printing drum 102 is supplied with printing ink in a second color by the inner press roller 107. The outer circumferential periphery of the press drum 103 is provided with a print sheet clamp segment 109, for clamping a leading edge of a print sheet 108, which clamps the leading edge of the print sheet 108 transferred from a paper feed section 110 and release the leading edge of the print sheet 108 at a position in the vicinity of an inlet portion of a sheet discharge section 111.
Now, the stencil printing machine thus arranged operates as follows. A leading edge of a first stencil sheet, which has been made on the basis of image data in a first color of an original is clamped with the sheet clamp segment 101b of the first printing drum 101, and a leading edge of a second stencil sheet, which has been made on the basis of image data in a second color of the original, is clamped with the sheet clamp segment 102b of the second printing drum 102, with the stencil sheets being mounted onto the outer circumferential peripheries of the respective screens 105.
Next, the first and second printing drums 101 and 102 and the press drum 103 are rotated in synchronism with one another in directions as shown by arrows in FIG. 6, thereby causing the print sheet 108 to be transferred between the first printing drum 101 and the press drum 103 from the paper feed section 110. The print sheet 108 thus transferred is clamped with the print sheet clamp segment 109 of the press drum 103, allowing the print sheet to pass along the outer circumferential periphery of the press drum 103 between the first printing drum 101 and the press drum 103. During this passing step of the print sheet, the inner press roller 106 is brought into press engagement with the screen 105 of the first printing drum 101 which is consequently expanded outward, allowing printing ink to be transferred to the print sheet 108 to reproduce a desired image pattern with a first color via a perforated image area of the first stencil sheet.
The print sheet 108, which has passed between the first printing drum 101 and the press drum 103, then passes between the second printing drum 102 and the press drum 103. During this passing step of the print sheet, the inner press roller 107 is brought into press engagement with the second screen 105 which is consequently expanded outward, allowing printing ink to be transferred to the print sheet 108 to reproduce a desired image pattern with a second color via a perforated image area of the second stencil sheet. As the sheet clamp segment 109 of the press drum 103 is rotated to a position near the inlet of the sheet discharge section 111, the sheet clamp segment 109 is released, with the released print sheet 108 being discharged to the given discharge position by the sheet discharge section 111. In this manner, two-color printing is completed.
A second prior art stencil printing machine is shown in FIG. 7, which is a schematic view of a part of the structure of the stencil printing machine. In FIG. 7, first and second printing drums 101 and 102 are rotatably supported in a frame body (not shown), and outer circumferential peripheries of the first and second printing drums 101 and 102 carry thereon stencil clamping bases 101a and 102a, respectively, which support thereon sheet clamping segments 101b and 102b for clamping stencil sheets (not shown) onto the stencil clamping bases 101a and 102a, respectively.
Further, the outer circumferential peripheries, except the first and second sheet clamping bases 101a and 102a, of the first and second printing drums 101 and 102 are constructed by respective porous structures that allow printing ink to permeate. Inside the first and second printing drums 101 and 102, respective printing ink supply means are located to supply printing ink, in first and second colors, to the outer circumferential peripheries of the first and second printing drums, respectively.
Also, press rollers 120 and 121 are rotatably supported in close proximity to the first and second printing drums 101 and 102, respectively.
The press rollers 120 and 121 are moveable between a wait position (shown by a solid line in FIG. 7) in which the press rollers 120 and 121 are held out of press engagement with the outer circumferential peripheries of the first and second printing drums 101 and 102, respectively, and a press engagement position (shown by a phantom line in FIG. 7) in which the press rollers 120 and 121 are brought into press engagement with the outer circumferential peripheries of the first and second printing drums, respectively. During printing operation, the inner press rollers 120 and 121 are maintained in the press engagement position. A paper feed section 122 is arranged to supply a print sheet 108 between the first printing drum 101 and the press roller 120, and a sheet discharge section 123 receives the print sheet 108 discharged between the second printing drum 102 and the press roller 121, thereby discharging it to a given position.
Now, the stencil printing machine thus arranged operates as follows. The leading edge of the stencil sheet made on the basis of image data in a first color and the leading edge of the stencil sheet made on the basis of image data in a second color are clamped with the sheet clamp segment 101b of the first printing drum 101 and the sheet clamp segment 102b of the second printing drum 102, respectively, causing the stencil sheets to be mounted onto the outer circumferential peripheries of the respective printing drums. Next, the first and second printing drums 101 and 102 and the press rollers 120 and 121, which are held in the press engagement position, are rotated in synchronism with one another in directions as shown by arrows in FIG. 7, thereby causing the print sheet 108 to be transferred between the first printing drum 101 and the press roller 120 from the paper feed section 122. The print sheet 108 thus transferred is imparted with a transfer force with rotation of the first printing drum 101 and the press roller 120 and passes between the first printing drum 101 and the press roller 120. During such a passing step of the print sheet 108, printing ink in the second color is transferred to the print sheet 108 via a perforated image area of the stencil screen (not shown). The print sheet 108, which has passed between the first printing drum 101 and the press roller 120, is further transferred between the second printing drum 102 and the press roller 121, with the print sheet 108 being imparted with a transfer force with rotation of the first printing drum 101 and the press roller 120 such that the print sheet 108 passes between the second printing drum 102 and the press roller 121. During traveling of the print sheet, the print sheet 108 is pressed with the press roller 121 such that printing ink in the second color is reproduced on the print sheet 121 in a desired image. Finally, the print sheet 108, which has passed between the second printing drum 102 and the press roller 121, is discharged with the sheet discharge section 123 into the given position. Thus, the two-color printing operation is completed.
By the way, in the first and second prior art stencil printing machines, printing in a single-color printing mode with the use of either one of plural printing drums encounters problems as described below. That is, in the first prior art practice, a perforated stencil sheet, which has been made in a stencil making step, is mounted only onto a selected one of the printing drums for printing with a selected color, i.e., for example, only onto the first printing drum 101 whereas the stencil sheet is not mounted onto the non-selected printing drum and the inner press roller 107 in the wait position, thereby executing printing operation to perform the single color printing.
However, the print sheet 108, whose leading edge is clamped with the sheet clamp segment 109, is transferred along the outer circumferential periphery of the press drum 103 and a trailing edge of the print sheet 108 is caused to separate from the outer circumferential periphery of the press drum 103 and is brought into contact with the screen 105 whereby the print sheet 108 is stained with ink. Although the non-selected printing drum, i.e., for example, the second printing drum 102 may be taken out from the frame body and the printing operation may be carried out to obtain a stainless single-color printing, troublesome work is undesirably required for taking out the non-selected printing drum from the frame body.
On the other had, in the second prior art stencil printing machine, since each of the press rollers 120 and 121 forms part of the sheet transfer means for the print sheet 108, the press roller 121 associated with the non-selected printing drum, i.e., for example, the second printing drum 102 should also be maintained in the press engagement position. However, when the press roller 121 associated with the non-selected printing drum, i.e., the printing drum 102, remains in the press engagement position, a large quantity of printing ink is necessarily adhered to both the press roller 121 and the print sheet 108. Consequently, it is impossible to carry out the single-color printing without troublesome work such as replacement of the non-selected printing drum with another printing drum on which printing ink is not adhered. Also, similar issues are encountered in other stencil printing machines where more than three printing drums are employed and it is contemplated to perform printing with the number of printing colors less than the number of the printing drums.
The present invention has been made to address various issues encountered in the prior art practices and has an object of the present invention to provide a stencil printing machine which enables printing in printing colors less in number than that of printing drums in an easy manner without causing any troublesome work.
According to a first aspect of the present invention, there is provided a stencil printing machine comprising a plurality of printing drums each formed with a porous structure to allow printing ink to permeate, a stencil making unit for making perforated stencil sheets each having a perforated image area formed on the basis of a color original and selectively making a non-perforated stencil sheet, with the perforated stencil sheets being supplied to and mounted on the printing drums, respectively, and a rotary printing press member located in close proximity to outer circumferential peripheries of the printing drums. A print medium is fed between the printing drums and the rotary printing press member such that the print medium is exerted with given printing pressure to cause the printing ink to permeate through perforated image areas of the stencil screens to transfer the printing ink onto the print medium with a desired image pattern. The stencil making unit allows the perforated stencil sheet to be mounted onto selected one of the printing drums, while allowing the non-perforated stencil sheet to be mounted onto the non-selected remaining one of the printing drums, thereby preventing the printing ink to permeate from the remaining one of the printing drums onto the print medium.
According to a second aspect of the present invention, there is provided a stencil printing machine comprising a frame body, a plurality of printing drums rotatably supported in the frame body and each including cylindrical porous structures to allow printing ink to permeate, a stencil making unit for making a perforated stencil sheet having a perforated image area formed on the basis of a color original and selectively making a non-perforated stencil sheet, with the perforated stencil sheet being supplied to and mounted onto the selected one of the printing drums, ink supply means for supplying printing ink to the plurality of printing drums, a rotary printing press member rotatably supported in the frame body in the vicinity of outer circumferential peripheries of the printing drums, and a paper feed section located in the frame body in the vicinity of the rotary printing press member for supplying a print medium between the printing drums and the rotary printing press member to allow printing ink to be transferred from the printing drums through the perforated image area to reproduce a desired image on the print medium. The stencil making unit allowing the perforated stencil sheet to be mounted onto selected one of the printing drums while allowing the non-perforated stencil sheet to be mounted onto the remaining non-selected printing drum, thereby preventing the printing ink to permeate from the remaining non-selected printing drum onto the print medium.
According to a third aspect of the present invention, there is provided a method of controlling a stencil printing machine having a plurality of printing drums each having a cylindrical porous structure to allow printing ink to permeate, and a rotary printing press member located in the vicinity of outer circumferential peripheries of the printing drums. The method comprises making a perforated stencil sheet having a perforated image area formed on the basis of a color original and a non-perforated stencil sheet, mounting the perforated stencil sheet onto selected one of the printing drums, mounting the non-perforated stencil sheet onto the non-selected printing drum for non-printing, supplying printing ink to the plurality of printing drums, and supplying a print medium between the printing drums and the rotary printing press member to allow printing ink to be transferred from the selected one of the printing drums through the perforated image area of the perforated stencil sheet to the print medium to reproduce a desired image thereon. During printing, the non-perforated stencil sheet is mounted on the non-selected printing drum to prevent printing ink to be transferred to the print medium.